There are three types of conventional apparatus for detecting a tire pressure.
The first type of apparatus, which is disclosed for example in the U.S. Pat. No. 4,567,459, includes a pressure detecting circuit which is mounted in the tire for generating an electrical signal indicative of the tire pressure.
The pressure detecting circuit is comprised of various electronic devices for frequency modulating the generated electrical signal. The modulated electrical signal is transferred to the vehicle body by electromagnetic coupling.
The second type is disclosed in page 15 of AUTOMOTIVE ELECTRONIC NEWS which was published on Jun. 26, 1989. This second type discloses a pressure detecting circuit for detecting the tire pressure mounted in the tire. The pressure detecting circuit includes a radio transmitting circuit for transmitting the detected signal to the body of the vehicle.
The third type is disclosed in Toku-Kai-Sho 61-141098 (Japanese laid open patent application). The apparatus includes a semiconductor pressure sensor and an LC resonant circuit both of which are mounted in the tire. The resistance of the semiconductor pressure sensor is changed according to the tire pressure. Since the Q value of the LC resonant circuit is changed in accordance with the resistance of the semiconductor sensor, the tire pressure is detected by detecting the Q value.
However, the above described conventional apparatus have problems of durability because the electronic devices or the circuit elements are mounted in the tire and therefore exposed to very severe conditions of high temperature and high vibration. Their characteristics are easily changed and deteriorated under such a condition. For example, the resistance of the semiconductor pressure sensor, and hence the Q value of the RLC resonant circuit disclosed in the above Toku-Kai-Sho 61-141,098 is easily changed by the tire temperature. In addition, these devices have an accuracy problem due to the relative difficulty of determining the Q value and due to the temperature dependence of the resistor.